they cannot impart any novel knowledge apart from themselves

It's not knowledge, it's skill at self-control and self-awareness. And like most other skills (riding a bicycle, driving a car, etc.) you can't acquire them by reading about them or simply thinking that you already know how to do them.

One of the most pernicious biases of the human brain -- pernicious because it interferes with self-improvement -- is that your brain believes it can always intuitively predict its own responses to mental and physical actions that it has never actually taken.

This means that, even when a self-improvement book includes a technique that produces some useful, novel result, most people will never actually try it, versus just reading about it and imagining that they know what it would have been like if they'd tried it... and concluding that it wouldn't do anything!

And meditation is absolutely in this category. There's a world of difference between intellectually "knowing" how much dreck your brain is putting out, and the practical experience of sitting there and listening to it, day after day, and realizing just how utterly stupid you are... it's also an active discouragement from listening to your own crap the rest of the day, too.

We could also get into health benefits, improved concentration, and all that sort of thing. From personal experience back when I was regularly attending the Dallas Zen center, it also makes you calmer, friendlier, and more confident, although alas those effects are not permanent if you stop. It's sort of more like exercise that way, I think.

Anyway, enlightenment is hardly a requirement for doing meditation, and Zen masters routinely discourage students from paying attention to any exotic or "spiritual" experiences they may have, precisely because the practice is an exercise, not merely a way of getting to some particular destination.